linux-kernel - Re: [PATCH v4 3/3] mm/mempolicy: introduce MPOL_WEIGHTED

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Date: Wed, 31 Jan 2024 21:18:50 -0500
From: Gregory Price <gregory.price@...verge.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Gregory Price <gourry.memverge@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
	corbet@....net, akpm@...ux-foundation.org, honggyu.kim@...com,
	rakie.kim@...com, hyeongtak.ji@...com, mhocko@...nel.org,
	vtavarespetr@...ron.com, jgroves@...ron.com,
	ravis.opensrc@...ron.com, sthanneeru@...ron.com,
	emirakhur@...ron.com, Hasan.Maruf@....com, seungjun.ha@...sung.com,
	hannes@...xchg.org, dan.j.williams@...el.com,
	Srinivasulu Thanneeru <sthanneeru.opensrc@...ron.com>
Subject: Re: [PATCH v4 3/3] mm/mempolicy: introduce MPOL_WEIGHTED_INTERLEAVE
 for weighted interleaving

On Thu, Feb 01, 2024 at 09:55:07AM +0800, Huang, Ying wrote:
> Gregory Price <gregory.price@...verge.com> writes:
> > -       u8 __rcu *table, *weights, weight;
> > +       u8 __rcu *table, __rcu *weights, weight;
> 
> The __rcu usage can be checked with `sparse` directly.  For example,
> 
> make C=1 mm/mempolicy.o
> 

fixed and squashed, all the __rcu usage i had except the global pointer
have been used.  Thanks for the reference material, was struggling to
understand that.

> > task->mems_allowed_seq protection (added as 4th patch)
> > ------------------------------------------------------
> >
> > +       cpuset_mems_cookie = read_mems_allowed_begin();
> >         if (!current->il_weight || !node_isset(node, policy->nodes)) {
> >                 node = next_node_in(node, policy->nodes);
> 
> node will be changed in the loop.  So we need to change the logic here.
> 

new patch, if it all looks good i'll ship it in v5

~Gregory


diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index d8cc3a577986..4e5a640d10b8 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1878,11 +1878,17 @@ bool apply_policy_zone(struct mempolicy *policy, enum zone_type zone)

 static unsigned int weighted_interleave_nodes(struct mempolicy *policy)
 {
-       unsigned int node = current->il_prev;
-
-       if (!current->il_weight || !node_isset(node, policy->nodes)) {
-               node = next_node_in(node, policy->nodes);
-               /* can only happen if nodemask is being rebound */
+       unsigned int node;
+       unsigned int cpuset_mems_cookie;
+
+retry:
+       /* to prevent miscount use tsk->mems_allowed_seq to detect rebind */
+       cpuset_mems_cookie = read_mems_allowed_begin();
+       if (!current->il_weight ||
+           !node_isset(current->il_prev, policy->nodes)) {
+               node = next_node_in(current->il_prev, policy->nodes);
+               if (read_mems_allowed_retry(cpuset_mems_cookie))
+                       goto retry;
                if (node == MAX_NUMNODES)
                        return node;
                current->il_prev = node;
@@ -1896,8 +1902,14 @@ static unsigned int weighted_interleave_nodes(struct mempolicy *policy)
 static unsigned int interleave_nodes(struct mempolicy *policy)
 {
        unsigned int nid;
+       unsigned int cpuset_mems_cookie;
+
+       /* to prevent miscount, use tsk->mems_allowed_seq to detect rebind */
+       do {
+               cpuset_mems_cookie = read_mems_allowed_begin();
+               nid = next_node_in(current->il_prev, policy->nodes);
+       } while (read_mems_allowed_retry(cpuset_mems_cookie));

-       nid = next_node_in(current->il_prev, policy->nodes);
        if (nid < MAX_NUMNODES)
                current->il_prev = nid;
        return nid;
@@ -2374,6 +2386,7 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
                struct page **page_array)
 {
        struct task_struct *me = current;
+       unsigned int cpuset_mems_cookie;
        unsigned long total_allocated = 0;
        unsigned long nr_allocated = 0;
        unsigned long rounds;
@@ -2391,7 +2404,13 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
        if (!nr_pages)
                return 0;

-       nnodes = read_once_policy_nodemask(pol, &nodes);
+       /* read the nodes onto the stack, retry if done during rebind */
+       do {
+               cpuset_mems_cookie = read_mems_allowed_begin();
+               nnodes = read_once_policy_nodemask(pol, &nodes);
+       } while (read_mems_allowed_retry(cpuset_mems_cookie));
+
+       /* if the nodemask has become invalid, we cannot do anything */
        if (!nnodes)
                return 0;